Twenty-two antigens have been identified as part of, or related to, the Kell blood group system. This genetic system is important from a clinical point of view, for incompatibility involving Kell group antigens can cause severe hemolytic transfusion reactions or hemolytic disease in newborn infants. In addition one of the variant Kell system phenotypes named McLeod, is associated with cell membrane abnormalities. The McLeod phenotype has an X-linked mode of inheritance and the red cells are characterized by a marked weakening of all Kell antigens, the absence of a "universal" antigen called Kx, acanghocytosis and reduced in vivo survival. McLeod subjects also have a progressive muscular dystrophic condition and neurological defects. A link between the McLeod syndrome and chronic granulomatous disease (CGD) has also been noted since normal leukocytes have Kx antigen whereas leukocytes of male patients with CGD do not. An understanding of the defect in McLeod red cells and its associated diseases requires an understanding of the biochemical nature of the Kell blood group antigens. We have recently shown that a 93,000 dalton glycoprotein, which is partially exposed on the surface of human red cells and is in physical association with other membrane proteins, carries several Kell blood group antigens. We now propose to: 1) characterize the 93,000 dalton protein in different Kell phenotypes with emphasis on the amino acid and carbohydrate composition of the exposed portions of the protein and 2) identify the other membrane proteins which are associated with the 93,000 dalton glycoprotein and 3) study the nature of the Kell complex in normal and variant Kell phenotypes.